Ethernet is an internationally recognized standard for communications between networked equipment, which is described in the IEEE 802.3 standard. Power over Ethernet is described for the Ethernet MDI covering multi-conductor twisted pair cabled links.
A Power over Ethernet (PoE) system includes power-sourcing equipment (PSE) and at least one powered device, referred to herein as an Ethernet Powered Device (EPD). The EPD is connected to the PSE through standard network cables, which transfer both power and data communications.
The 802.3 IEEE Standard provides guidelines and requirements for PoE covering PSE and EPD design. This standard includes limits for PSE's and EPD's that address EPD detection by the PSE, power classification of the EPD, and powered operating characteristics. The updated 802.3 IEEE Standard 802.3at expanded the physical layer classification to include Two-Event classification and it added data-link classification. PSE's and Class 4 EPD's with these capabilities are referred to as Type 2. Two-Event classification and data-link classification protocols include mutual identification, where an EPD and a PSE may both learn the other's Type and power requirements and may adjust power consumption or allocation accordingly.
In order for the higher power levels of Class 4 under 802.3at to be used, one of the two classification protocols must first enable the higher power level. Two-event classification has specified timings and two class-pulse components that are used by the PSE to both classify an EPD and to indicate to the EPD that full class 4 power will be available from the PSE. If a PSE does not provide two event classification, this indicates that the PSE is not allowing higher power, which could occur if the PSE is Type 1, or if it isn't able to supply the higher power, or it might only allocate higher power via the Data link power classification. Data-Link Classification is a power negotiation protocol, which provides power allocation adjustment capability, based upon communications exchanged between a PSE and an EPD. 802.3at specified power over two of the 4 pairs present in most Ethernet twisted pair connections. 802.3bt will add 4 pair powering and higher power levels. Classification protocols may also be introduced and/or extended.
When an EPD is powered by a PSE, the power consumption is subject to minimum and maximum power level limits as determined by the power classification. Associated with these limits are timings, where peak, average, and minimum power consumption levels are timing dependent. For instance, an average power is based upon 1-second averaging. An EPD's power consumption can be highly dependent upon operating state or activity. This can complicate the testing of EPD's for compliance and for determining power classification requirements, particularly for data-link classifications, where the settings may be of a much higher resolution than physical layer classifications. The IEEE 802.3 clause 33, Power over Ethernet technology is well established and widely deployed in devices operating on local area networks. The popularity of these devices is related to the abundance of low cost equipment and easily installed structured cabling components utilized to provide connectivity and power to networked devices.
Assessing the physical layer performance characteristics of an IEEE 802.3 clause 33 is an important step in the development and manufacture of Ethernet powered devices because it provides requirements for the safety and interoperability of such devices. Because of the cost and complexity of traditional physical layer testing methods and solutions, many producers of products with one or more Ethernet ports rely on commercial Ethernet power sourcing and/or powered device equipment for assessing such devices.
However, interoperability testing offers very limited parametric insight meaning it cannot assure tested devices will perform properly under all network interface conditions. As a general matter in testing Ethernet powered devices, the assessment of device performance is a challenging task owing to the need to precisely stimulate the device while measuring detection and classification parameters and power consumption over various operating conditions. Although the concept of powering devices over network cabling is simple, testing the EPD functionality is complex.
Similarly, device operation by the test equipment is impractical owing to the fact that some of the devices are operationally complex and may require expensive and complex support equipment to place the device into an operational mode. The task of parametrically testing Ethernet powered devices using conventional means has been expensive, laborious, and invasive, often defying highly automated approaches. For this reason, the network equipment industry often relies solely on interoperability testing to qualify interface performance.